Plastic materials such as polymeric films have been widely used for packaging various food and non-food products. In order to ensure proper preservation of products packaged in such polymeric films, it is necessary to provide the films with barriers against transmission of air, moisture, deleterious flavors, etc. Unmodified polymeric films, however, typically lack sufficient gas and moisture barrier characteristics needed for proper packaging requirements. For example, polypropylene films are particularly preferred in the manufacture of packaging films due to their low cost and ease of manufacture. Such films, however, inherently permit the transmission of oxygen and water vapor from the outside of the film to the inside of the package made up of the film. As will be recognized by those skilled in the art, transmission of oxygen and water vapor through food packagings promote deterioration of the foods packaged therein.
Multi layer polymeric films have been developed having improved gas and moisture barrier characteristics. For example, it is known that films incorporating ethylene vinyl alcohol as a co-extruded coated layer thereon display improved oxygen and moisture barrier properties. U.S. Pat. No. 5,192,620 discloses an oriented polypropylene base layer having a coating on one surface of a blend of a vinyl alcohol homopolymer or copolymer and an ethylene-acrylic acid copolymer, with a further metal layer thereon. Films incorporating ethylene vinyl alcohol as co-extruded or coated layers, however, are difficult and expensive to manufacture.
The use of multi layer films incorporating metal layers has also been proposed for providing improved barrier properties. U.S. Pat. No. 5,194,318, the disclosure of which is incorporated herein by reference, discloses a metallized oriented thermoplastic film combination having a propylene homopolymer or copolymer substrate with a high density polyethylene skin layer on at least one side of the substrate. The high density polyethylene skin layer further includes a thin metal layer which has been vapor deposited thereon. In one embodiment, the high density polyethylene skin layer is flame or corona discharge treated, and the metal coating is thereafter deposited onto the flame or corona discharge treated layer. Although films which incorporate a high density polyethylene skin do not suffer from the deleterious effects of metal pick off, such films do not provide optimum oxygen and water vapor barrier properties for all packaging needs.
Plasma treatment of polymeric films with hydrocarbons has been proposed to impart improved adhesion of metal coatings to films. For example, U.S. Pat. No. 4,897,305 discloses a packaging film of a polyolefin substrate such as polypropylene having a thin layer of a low molecular weight hydrocarbon and a metal layer thereon. The film is prepared by exposing the polypropylene substrate to a plasma treatment of aliphatic hydrocarbon vapors, followed by vapor deposition of a metal layer. The films produced with plasma treatment involving aliphatic hydrocarbons, however, are expensive to manufacture, and the hydrocarbon layer is not believed to add any significant barrier properties to the film.
While the prior art discloses various packaging films, such films are typically difficult to manufacture, involve incorporation of costly materials and processes, and provide inefficient adhesion for a subsequent vapor deposited metal layer. Accordingly, a need exists for a packaging film which is simple and inexpensive to manufacture and which is capable of providing enhanced barrier properties to oxygen and water vapor transmission.